Viruses, bacteria, fungi, and parasites populate diverse ecological niches from the human body to the extreme conditions of arctic ice or thermal vents in the deep ocean.

The human body contains more bacteria than human cells. In fact, our lives would not be possible without them. The gut, for example, contains a dense bacterial ecosystem, with numbers as high as 100 trillion bacteria in total. This ecosystem assembles from birth through the first weeks of life. These microbiota manufacture some essential vitamins, provide us with nutrients, help to educate our immune systems, and crowd out potentially harmful species.

Various bacteria and fungal species are also responsible for fermentation in food, environmental remediation, and breakdown of organic waste.

Relatively few species are outright pathogens – organisms that trigger acute or chronic disease. However, some species that are harmless in the gut or on the skin can cause problems if they invade the blood or vital organs.